1. Field of the Invention
This invention relates to nucleic acid markers for a locus that controls apomixis and vectors containing the DNA. The markers are also useful for marker-assisted selection in conventional crossing programs.
2. Description of the Related Art
Reproduction in plants is ordinarily classified as sexual or asexual. The term apomixis is generally accepted as the replacement of sexual reproduction by various forms of asexual reproduction (Rieger et al, IN Glossary of Genetics and Cytogenetics, Springer-Verlag, New York, NY., 1976). Apomixis is a genetically controlled method of reproduction in plants where the embryo is formed without union of an egg and a sperm. There are three basic types of apomictic reproduction: 1) apospory-embryo develops from a chromosomally unreduced egg in an embryo sac derived from the nucellus, 2) diplospory- embryo develops from an unreduced egg in an embryo sac derived from the megaspore mother cell, and 3) adventitious embryony- embryo develops directly from a somatic cell. In most forms of apomixis,2 pseudogamy or fertilization of the polar nuclei to produce endosperm is necessary for seed viability. These types of apomixis have economic potential because they can cause any genotype, regardless of how heterozygous, to breed true. It is a reproductive process that bypasses female meiosis and syngamy to produce embryos genetically identical to the maternal parent. With apomictic reproduction, progenies of specially adapted or hybrid genotypes would maintain their genetic fidelity throughout repeated life cycles. In addition to fixing hybrid vigor, apomixis can make possible commercial hybrid production in crops where efficient male sterility or fertility restoration systems for producing hybrids are not known or developed. Apomixis can make hybrid development more efficient. It also simplifies hybrid production and increases genetic diversity in plant species without good male sterility systems.
In sexual reproduction, usually a megaspore mother cell arising from the hypodermal layer of the ovule enlarges and goes through meiosis to form a linear tetrad of megaspores each with a haploid chromosome number. The three micropylar spores degenerate while the functional chalazal spore enlarges to form an embryo sac with an egg, two polar nuclei, two synergids, and three antipodals.
In apospory, a megaspore mother cell may begin enlarging and even produce chromosomally reduced megaspores but this sexual tissue usually degenerates before embryo sac development. Instead, somatic cells of the nucellus enlarge and the nuclei of these cells go through mitotic divisions to form one to many embryo sacs per ovule each with one to eight chromosomally unreduced nuclei. Aposporous apomicts are characterized by the participation of one or more nucellar cells in the direct formation of one or more embryo sacs. Each nucleus of the aposporous embryo sac has the somatic chromosome number and genotype of the maternal plant. Some aposporous species, pseudogamous apomicts, require pollination and fertilization of polar nuclei for the development of endosperm, but the unreduced aposporous egg develops without fertilization (parthenogenetically). Female meiosis usually is disturbed in aposporous apomicts that form all of their seed asexually (obligate apomicts) so that no functional megaspore continues development beyond the first mitotic division. Facultative apomicts exist in which meiosis and aposporous development occur simultaneously and both reduced and unreduced embryo sacs ultimately reside in the same individual and/or the same ovule. Thus, the two modes of reproduction, sexual and asexual, can coexist or one can be dominant over the other. During obligate apospory, several events must be coordinately regulated, i.e., disturbance or failure of meiosis, aposporous embryo sac development, parthenogenesis; nevertheless only one or a few genes may be responsible for the cascade of events. Some genetic studies suggest that aposporous apomixis is simply inherited (Asker et al, Apomixis in Plants, CRC Press, 1992; Nogler, Embryology of Angiosperms, B. M. Johri, Ed., Springer-Verlag, 475-518, 1984; Winkler, Progr. Rei. Bot., Vol. 2, 293, 1908).
The main difference in diplospory compared to sexual development is that a single megaspore is derived from the megaspore mother cell without meiosis, thus this megaspore has the somatic chromosome number. An embryo sac similar in appearance to a sexual embryo sac develops, but with an egg containing the somatic chromosome number.
In adventitious embryony, embryos develop directly from somatic cells of the ovule without formation of embryo sacs. Sexual sacs which are needed for endosperm formation may also form in the same ovule.
Introducing the apomictic trait into normally sexual crops has been attempted. Asker (Heredias, Vol. 91, 231-240, 1979) reports that attempts have been unsuccessful with wheat, sugar beets, and maize. PCT publication WO 89/00810 (Maxon et al, 1989) discloses inducing an apomictic form of reproduction in cultivated plants using extracts from nondomesticated sterile alfalfa plants. The PCT discloses that a soybean hybrid was produced applying this extract to the soybean which was male sterile through the F.sub.4 generation. The publication further discloses that corn treated with the extract displayed a sterility conversion of 15-26% for seven of the eight genotypes treated. When induction of male sterility was evaluated in sorghum, sunflower, pearl millet, and tomato it was reported that there was reduced seed set in sorghum, pearl millet, and sunflower and reduced fruit set in tomato.
It would be ideal to find genes controlling obligate or a high level of apomixis in the cultivated species and be able to readily hybridize cross-compatible sexual x apomictic genotypes to produce true-breeding F.sub.1 hybrids. In reality, most desirable genes controlling apomixis are found in the wild species which are distantly related to the cultivated species. Although interspecific crosses may be possible between the cultivated and wild species, chromosome pairing between genomes is usually low or nonexistent.
Although apomixis is effectively used in Citrus to produce uniform and disease- and virus-free rootstock (Parlevliet JE et al, in Citrus. Proc. Am. Soc. Hort. Sci., Vol. 74, 252-260, 1959) and in buffelgrass (Bashaw, Crop Science, Vol. 20, 112, 1980) and Poa (Pepin et al, Crop Science, Vol. 11, 445-448, 1971) to produce improved cultivars, it has not been successfully transferred to a cultivated crop plant. The transfer of apomixis to important crops would make possible development of true-breeding hybrids and commercial production of hybrids without a need for cytoplasmic-nuclear or chemically-induced male sterility and high cost, labor-intensive production processes. An obligately apomictic F.sub.1 hybrid would breed true through the seed indefinitely and could be considered a vegetative or clonal method of reproduction through the seed. The development of apomictically reproducing cultivated crops would also provide a major contribution toward the food security in developing nations (Wilson et al, IN Proceedings of the International Workshop on Apomixis in Rice, Changsha, People's Republic of China, Jan. 13-15, 1992. Hunan Hybrid Rice Research Center, Changsha, People's Republic of China)
Genes involved in apomictic reproduction have not been cloned to date. Therefore, nucleic acid markers tightly linked to the apomixis trait are useful for obtaining the gene(s) which are involved in apomictic reproduction. These markers are also useful for identifying cultivated hybrid plants having apomictic reproduction. The present invention, described below, provides markers which are tightly linked to the apomictic trait which are different from the related art.